New and more powerful electronic devices and computers are continually developed (e.g., digital audio players, video players, cell phones, personal digital assistants). As more new features are added into electronic devices and computers, there continues to be pressure in integrating and reducing the size of components within electronic devices. Particularly, the acoustic input components of electronic devices and computers are also subject to this size-reducing pressure. A conventional Electret Condenser Microphone (ECM) is an electro-mechanical component that has been used as an acoustic input component in electronic devices for many years. Even though the sizes of conventional ECMs have been reduced substantially (e.g., 4×1.5 mm), it is approaching its fundamental physical size limit.
MEMS (Micro Electro Mechanical Systems) technology has enabled the manufacturing of microfabricated microphones by utilizing robust processes from the semiconductor industry. Microfabricated microphones offer many advantages over traditional ECMs such as: substantial reduction in size, wider operational temperature ranges, more tolerance to moisture, lower manufacturing cost, compatibility with auto pick-and-place tools and standard reflow processes in installation and etc.
A microfabricated microphone generally consists of a flexible diaphragm and an electrically charged back-plate with damping holes. The diaphragm and the back-plate form a capacitor. Sound pressure can then dynamically deform the diaphragm to change the capacitance of the capacitor, and thus sound is transformed into electrical signals. In a conventional microfabricated microphone, all or substantially all edges of the diaphragm are mechanically fixed to the substrate in one form or another. This structural design prevents the residual stresses in the diaphragm thin film from relaxing. Residual stress in the sensing diaphragm can dominate the diaphragm's mechanical performance and, for example, reduce sensitivity with increasing residual tensile stress or lead to undesirable buckling of the diaphragm with increasing compressive stress. Thus, residual stress in the diaphragm can negatively affect a microfabricated microphone's sensibility, noise, and over-pressure response.
There are several conventional remedies to mitigate the effect of the residual stress on the mechanical behavior of the sensing diaphragm. One remedy is controlling residual stress to very low magnitudes. However, controlling residual stress requires very tight process control for consistent stress. Further, the mechanical behavior of the sensing diaphragms is usually dominated by the residual stress even within practical levels of residual stress. A more effective remedy is minimizing the effect of stress on diaphragm mechanics through mechanical designs. One method of minimizing the effect of residual stress through mechanical design is the ‘free plate’ scheme. (See Loeppert et al., U.S. Pat. No. 5,490,220). In the free plate scheme, the sensing diaphragm is largely free at the edges, with the exception of connection at a portion of the edge to a narrow arm, which is necessary for electrical connection to the diaphragm. Since the sensing diaphragm is mostly not attached to the substrate at its peripheral diameter, it allows the residual stress to relax through radial contraction or expansion of the diaphragm. However, the arm connected at a portion of the edge introduces radial and angular asymmetry in the sensing diaphragm structure, and as a result asymmetry in the stress relaxation. It is also necessary to mechanically confine the diaphragm to overcome the large compliance of the free plate attached at the end of the cantilever arm. Therefore, even though free plate scheme may mitigate some residual stress in the sensing diaphragm, there are still performance limits and complications in its manufacturing process.